The present invention is directed in general to a catalytic oxidation process of a carbonaceous feed stock and to a catalyst being useful in the process. In particular, the invention relates to preferential oxidation of carbon monoxide and other carbonaceous compounds in hydrogen containing feed stocks in presence of a catalyst comprising gold as the active catalytic component supported on a support comprising magnesium aluminum oxide.
Carbon monoxide and lower hydrocarbons, being present in a large number of industrial processes, are usually unwanted in the products from those processes. A number of processes for the removal of these compounds are known in the art. Thus, U.S. Pat. No. 5,662,873 and U.S. Pat. No. 5,693,302 disclose a process for the removal of CO mainly from inert gases using a gold/palladium, gold, and gold/silver or gold/platinum metal catalyst supported on titanium dioxide. By the above known processes also certain amounts of hydrogen are lost by oxidation.
When producing hydrogen, e.g. by steam reforming, it is necessary in a number of processes to remove residual amounts of CO from the product gas selectively without oxidation of hydrogen. One process to selectively remove CO is preferential oxidation, whereby CO is oxidized with oxygen to carbon dioxide. In the preferential oxidation process it is desirable that preference for CO oxidation is substantially higher than hydrogen oxidation to avoid loss of valuable hydrogen.
U.S. Pat. No. 5,955,395 discloses use of Pt/zeolite catalysts and U.S. Pat. No. 3,216,783 relates to Pt/Al2O3 catalysts for use in the abatement of CO. Oh and Sinkevick (S. H. Oh and R. M. Sinkewitch, J. Catal. 142, 254 (1993)) mention use of Ru/Al2O3 and Rh/Al2O3 catalysts for use in the selective CO removal, while U.S. Pat. No. 5,702,838 uses a catalyst comprising an A-type zeolite and at least one metal selected from the group consisting of Pt, Pd, Ru, Au, Rh and Ir, or an alloy of two or more metals. A limited number of catalyst systems based on gold on reducible metal oxide supports have been investigated for the oxidation of CO in the presence of H2: Au/MnOx (R. M. Torres-Sanchez, A. Ueda, K. Tanaka, M. Haruta, J. Catal. 168, 125-127 (1997)) and Au/Fe2O3 (M. J. Kahlich, H. A. Gasteiger, R. J. Behm, J. Catal. 182, 430-440 (1999)). The term xe2x80x9creduciblexe2x80x9d oxides is used in literature for those catalysts containing a transition metal ion that can occur in at least two different oxidation states (cf. M. Haruta, Gold as low temperature oxidation catalyst: factors controlling activity and selectivity, 3rd World Congress on Oxidation Catalysis, R. K. Grasselli, S. T. Oyama, A. M. Gaffney, J. E. Lyons, San Diego, Elsevier Science B. V., pages 123-134 (1997)) and it is distinguished between non-reducible and reducible oxides in the following according to this definition.
Generally, the use of gold catalysts for oxidation of CO by air or oxygen containing atmosphere is known in the art. Haruta et al. describe preparation of gold catalysts in U.S. Pat. No. 4,698,324 and the preparation of gold on reducible metal oxides by co-precipitation. The gold loading was very high (about 10% by weight) in U.S. Pat. No. 4,698,324 and improved methods were reported in e.g. U.S. Pat. No. 5,789,337. U.S. Pat. No. 5,550,093 mentions use of zeolites as hosts for gold, preferentially in combination with transition metal oxides. Preparation of small gold particles and deposition of gold colloids have been described in the open literature (J.-D. Grunwaldt, M. Maciejewski, O. S. Becker, P. Fabrizioli, A. Baiker, J. Catal. 186, 458 (1999)). However, strong dependence of the Co oxidation activity on the nature of the support material has been found and described in literature (M. Haruta, Catal. Today 36, 153(1997), J.-D. Grunwaldt, M. Maciejewski, O. S. Becker, P. Fabrizioli, A. Baiker, J. Catal. 186, 458 (1999)). Also the poisonous character of water on some Au/metal oxide catalysts has been mentioned (M. A. Bollinger, M. A. Vannice, Appl. Catal. B Environm. 8, 417 (1996), J.-D. Grunwaldt, C. Kiener, C. Woegerbauer, A. Baiker, J. Catal. 181, 223 (1999)). Usually, use of reducible oxides is preferred to achieve more active gold catalysts. Additionally, combinations of gold and Al2O3 and gold and MgO/Mg(OH)2 are known in the art. It is further known that the Au cluster size has to be about 1 nm on Be(OH)2 and Mg(OH)2 (M. Haruta, Gold as low temperature oxidation catalyst: factors controlling activity and selectivity, 3rd World Congress on Oxidation Catalysis, R. K. Grasselli, S. T. Oyama, A. M. Gaffney, J. E. Lyons, San Diego, Elsevier Science B. V., pages 123-134 (1997)).
Oxidation of CO and hydrocarbons by gold as noble metal on cobalt oxide (Co3O4) or ferric oxide (Fe3O4) as transition metal is mentioned in U.S. Pat. No. 5,895,772.
Several further oxidation reactions of carbonaceous compounds (i.e. alcohols, alkenes, alkines) as well as hydrogenation and water gas shift reactions are catalyzed by gold (M. Haruta Catalysis Today 36, 163 (1997), H. Sakurai, A. Ueda, K. Atsushi, M. Haruta, Chem. Commun., Issue 3, 271 (1997); D. Andreeva, T. Tabakova, V. Idakiev, P. Christov, R. Giovanoli, Appl. Catal. A: General 169, 9 (1998)).
However, in the above mentioned state of art it is nowhere disclosed to use gold on a simple non-reducible support of metal oxides as a catalyst for preferential oxidation of carbonaceous compounds in the presence of hydrogen.
It has now been found that gold on the non-reducible MgAl2O4 spinel support provides in general an improved catalyst for the above mentioned gold catalyzed processes, particularly in the oxidative removal of carbon monoxide. The catalyst has high stability in the presence of water and CO2 and shows selectivity towards the oxidation of carbon monoxide in presence of hydrogen.
Preparation and use of MgAl2O4 as support in different processes is known in the art, cf. J. Am. Cer. Soc., Discussion and Notes, Vol. 53(1), 56 (1970), J. Mater. Sci. Lett. 19, (2000) 1033; Mater. Lett. 31 (1997) 281, U.S. Pat. No. 5,997,835.
In accordance with the above findings and observations, this invention is a process for the oxidation of carbonaceous compounds by contacting the compounds with a catalyst comprising gold on a non-reducible support of MgAl2O4. MgAl2O4 is stable in an environment with hydrogen, carbon monoxide, carbon dioxide, methanol and water. Thus, the invention is in particular useful in the oxidation of carbon monoxide and of carbon monoxide in presence of hydrogen, water and carbon dioxide.
The catalyst according to the invention is additionally useful in selective oxidation of CO for gas purification purposes, oxidation of other carbonaceous compounds and water gas shift reaction.
The catalytic activity during oxidation of CO in case of the Au-MgAl2O4 system is maintained or even improved in the presence of water and/or CO2, and the catalyst material is selective towards CO oxidation in the presence of H2. Best results are obtained on the magnesium aluminum oxide catalysts, when the gold particles are smaller than 10 nm and in most cases calcinations between 200 and 600xc2x0 C. have been found to be advantageous. Both powder and preshaped carriers can be used.
In the preparation of gold based MgAl2O4 catalysts:
(a) porous ceramic carrier material containing MgAl2O4 is used (in the following only referred to as MgAl2O4); and
(b) a method for applying small gold particles on the ceramic carrier is employed.
This can be i.e. achieved by
(a) deposition of gold colloids on the support;
(b) homogeneous deposition precipitation of a Au(III) precursor, for instance Au(NH3)4(NO3)3; and
(c) chemical vapour deposition of a volatile Au(III) compound, for instance (CH3)2Au (CH3COCHCOCH3).
These and further methods for producing small gold particles are state of the art and described in the literature.
The carrier material MgAl2O4 is described in literature (J. Am. Cer. Soc., Discussion and Notes, 53(1), 56 (1970), J. Mater. Sci. Lett. 19, (2000) 1033; Mater. Lett. 31 (1997) 281) and an excess of Al is preferred. The specific surface area is preferentially high.
Several examples are given to illustrate the invention in further detail, but without restricting its preparation by other techniques aiming at the production of small gold particles on an MgAl2O4 support.